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Zone 3 roof dampening concern unvented attic

Mike Beckham | Posted in General Questions on

New home construction – roof design
asphalt shingle, 30# felt, OSB, 5 1/2″ open cell foam. Should I be concerned about the design causing moisture problems over time?

Mike

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    If you have a simple roof line, your best option is to install rigid insulation on the outside of the sheathing.

    Recent information suggested that open cell foam installed to the underside of roof sheathing is risky in all climate zones. See this article for details (https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing).

    If you went ahead, the 5.5 inches of open cell in your stack up would deliver about r-20, which is a long way from the code requirement of r-39. You would be closer to code with closed cell foam (about r-36), but you probably would want ventilation above the sheathing to encourage drying. (But let's see what other posters think.)

  2. D Dorsett | | #2

    Yes, you should be concerned.

    The vapor permeance of open cell foam at 5.5" isn't low enough to fully avoid wintertime moisture accumulation problems. With dark shingles in a sunny location it might be fine, but for a north facing or shaded pitch it could be an issue.

    If you installed a smart vapor retarder such as 2 mil nylon (eg Certainteed MemBrain) or a class-II vapor retarder such as "vapor barrier latex" on sheet rock on the interior side it would lower the risk by quite a bit.

    Alternatively, as little as R5 of insulating sheathing above the roof deck would be adequate dew point protection for a roof in zone 3, and would also provide a modest amount of thermal break over the rafters/trusses.

    If 5.5" of open cell foam is the only insulation in that roof it is only about R20- well below IRC code minimum R38 for zone 3. That too should also be of concern. Installing 3" of rigid polyisocyanurate or a 3.5" nailbase polyiso panel above the structural roof deck would bring it up to code-min, and would provide huge dew point margin from interior moisture drives.

  3. GBA Editor
    Martin Holladay | | #3

    Mike,
    I don't recommend the use of open-cell spray foam on the underside of roof sheathing. To learn why, read these two articles:

    High Humidity in Unvented Conditioned Attics

    Open-Cell Spray Foam and Damp Roof Sheathing

  4. Mike Beckham | | #4

    Thank you very much for your timely responses. I will meet with the builder and make changes to the design. I'm so glad I found this site and thank you for your time.
    Mike

    I will post the proposed wall assembly for your thoughts, as well.

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